Current fuses composed of a low-melting metal member of lead, tin, antimony or the like are commonly known as protective elements for cutting off over-current.
Protective elements that can be used to prevent not only for over-current but also overvoltage are also known, in which a heat-generating member and a low-melting metal member are layered in that order on a substrate, the heat-generating member generates heat in the event of overvoltage, and this heat blows out the low-melting metal member (Japanese Patent 2,790,433).
However, when an insulating layer is formed by screen printing in such a protective element, the mesh used in the screen printing makes the surface of the insulating layer uneven, and this unevenness has been indicated as a problem in that they hinder smooth, spherical segmenting during the heating of the low-melting metal member layered over the insulating layer. To deal with this problem, it has been proposed that the heat-generating member and the low-melting metal member be disposed in planar fashion on the substrate, with no insulating layer interposed in between them (Japanese Patent Applications Laid-Open Nos. H10-116549 and H10-116550).
However, disposing the heat-generating member and the low-melting metal member in planar fashion makes it impossible to produce a more compact element. Also, since here again the low-melting metal member is provided so as to be in solid contact with the substrate, the substrate inevitably hinders the flow of the low-melting metal member in a molten state, which means that smooth, spherical segmenting of the low-melting metal member cannot be guaranteed.
In view of this, it is an object of the present invention to ensure consistent spherical segmenting of the low-melting metal member during melting, in a protective element comprising a heat-generating member and a low-melting metal member on a substrate, and in which the low-melting metal member is heated and blown out by the heat generated by the heat-generating member.